An understanding of primary palate development is necessary for further progress to be made in research related to malformations such as cleft lip and palate. We have chosen the chick embryo as a model system because it permits studies on both cell proliferation and cell migration. Findings from this study will eventually be extended to normal and abnormal primary palate development in mammalian embryos. Embryos will be analyzed from the stages of development beginning with the invagination of the nasal placode, through the stages of formation of the nasal processes up to the time of fusion. Cell proliferation of mesenchyme will be measured by labelling indices, maximum labelling determinations, and by determination of fraction of labelled mitoses through the use of 3H-thymidine incorporation into DNA. These determinations will yield information on the length of the cell cycle, and of its component parts for each of the populations studied, and on the presence of subpopulations differing generation times. The areas to be studied included the lateral and medial nasal processes, the base of the olfactory placode, the maxillary process and the mesenchyme of adjacent areas. Cell migration into the nasal processes during their development will be studied by the use of transplants between Japanese quail and chick embryos and between 3H labelled donors and unlabelled hosts. The areas to be examined are the same as those listed for cell proliferation studies and pathways of migration as well as rates of migration will be determined.